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The Compellor

Article from Home & Studio Recording, May 1985

Already well-known for their Aural Exciter, Aphex have come up with another mystery box. If you need to compress, limit and level at once, then this is definitely the machine for you, but such sophistication doesn't come cheaply.

According to Aphex, the Compellor is a revolutionary audio processor that delivers intelligent compression levelling and peak limiting simultaneously. Apparently this is achieved using analogue computer technology that analyses the input waveform and then uses that information to control a single VCA in each signal channel. Basically, the Compellor has been developed to control the gain of a signal in such a way as to be completely unobtrusive.

At first glance, the Compellor might seem to be just another compressor limiter, but in reality its functions are more innovative and varied. Even the meters are novel, using dual coloured LEDs so that they can effectively display information relating to two parameters simultaneously. The average signal is handled by red LEDs, with green ones dealing with the higher peak levels.


Well, it's another 1U x 19" rack module, but then that is a very sensible format for any signal processor. The styling matches that of the Aural Exciter Type B with its blue-grey anodised front panel, and its general appearance is clean cut and upmarket, an image the Compellor in fact was found to deserve. Both the inputs and outputs are on balanced XLRs and the working level may be internally set to -10, 0, +4 or +8 dBm, thereby enabling the Compellor to be level compatible with virtually any professional or semi-professional system in common usage.


Before looking at the controls, it is necessary to examine some of the parameters involved in the gain control chain to understand exactly how they function.

As I understand it, the term 'levelling' refers to a long term control of signal level, the effect of which is to maintain a relatively constant average output level without responding to, and therefore affecting short term detail.

Compression, on the other hand, is a relatively short term process and the Compellor's compression circuitry has a ratio that can vary from 1.1:1 to 20:1. The attack and release times, are varied automatically according to how the circuitry interprets the incoming signal and the absence of a compression ratio control is due to the fact that it is automatically matched to the programme material.

Limiting on the other hand is more of a safety feature and its job is to clamp excessive signal levels before they can hit the end stops. Oscillographs show that the Compellor is capable of arresting an errant transient in just two microseconds - unbelievable.

The control circuitry itself contains two systems developed specially, one of which is the 'Dynamic Verification Gate' (DVC). This continuously monitors average levels of both long and short term signals, and by constantly comparing them, prevents the system carrying out unnecessary gain changes that could subjectively compromise programme dynamics.

Another feature of the DVC is that it inhibits the gain release during short pauses in the programme material that could otherwise produce noticeable 'pumping'. Heavily compressed vocals are particularly prone to this effect (by nature of their dynamics), but the Compellor has been designed to cope with this, producing a completely 'unprocessed' sound at all times.

Secondly, the 'Dynamic Recovery Computer', or DRC allows the gain recovery release time to be substantially reduced, depending upon the nature of the input signal. Short transients are recognised and treated in this way to prevent following low level sounds from being unduly compressed.


As previously stated, metering on the Compellor is something of an innovation. It can be switched to monitor either the line input or the line output. In addition, you can select the metering to display 'Program' or 'Gain Reduction'. In gain reduction mode, compression is shown as a moving green bar, giving a resolution of 2dB per division, and the amount of levelling is shown by a moving red segment on the same scale.

Conversely, when in program mode, the average VU is displayed as a red bar with the peak level simultaneously shown in green.


When you take into consideration the ingenious operating principles used by the Compellor, the controls used are actually surprisingly simple.

The input level control varies the amount of processing to be applied to any given signal. Maximum gain control occurs when this control is turned fully clockwise.

'Process Balance' allows the user to vary the ratio of levelling to compression used. When fully clockwise, this process consists of compression only, and when anti-clockwise, only levelling occurs. In either mode, or using any blend of the two, the peak limiter constantly ensures that no transients exceed a ceiling level, set 12dB above the nominal 0VU level.

The output gain is in reality make-up gain so that reduction within the system may be compensated for.

Also fitted is an in/out switch so that A/B comparisons can be made. This is achieved using a relay circuit so that when the Compellor is not powered up, the input is directly routed to the output. A bi-coloured status LED shows red when the Compellor is active and green when the Compellor is bypassed.

Next on the list comes the silence gate. Although its name seems to imply that it's simply a noise gate, it is in fact, an extremely sophisticated device.

In effect, the gate circuitry detects low level signals that fall below the level set by the threshold control and then holds the threshold release voltage voltage on a sample and hold circuit until the threshold is once again exceeded, whereupon time normal operation resumes. The result of this is that the compressor no longer brings the background noise up to programme level in silent passages when heavy compression is used. This proves to be very effective in maintaining a high signal to noise ratio without encountering any of the side effects usually associated with noise gates.

The stereo enhance control is nothing more than a in/out pushbutton with the obligatory status LED. This is not a stereo simulator and has no effect on mono material, but it does subtly enhance material that already exists in stereo form. Unfortunately, Aphex are none too forthcoming in their description of exactly how this functions but it is apparently a matrix system that modifies the action of the gain control sidechain circuitry.

Circuit Considerations

Aphex did however supply circuit diagrams for the Compellor, and from examining them, it's clear that a great deal of thought and care has gone into all aspects of the design. The input circuitry is electronically balanced using a true instrumentation amplifier circuit that offers both very good common mode rejection and closely matched input impedances for the hot and cold balanced inputs. RF rejection is provided by an LC input network and the input amplifier is followed by the gain control device (in this case the Aphex 1537 VCA chip - widely regarded as being one of the best gain control elements now available) The output stage is also a transformerless balanced circuit featuring push-pull bi-polar output stages capable of driving long lines and designed to survive short circuiting.

In Use

If you're looking for an effect, then this unit is not really for you. In fact it is generally impossible to detect that the signal is being treated at all until you do an A/B test and then realise that the programme dynamics are being kept firmly under control. Even under surprisingly high levels of compression, the Compellor remains virtually 'transparent' and only when everything is literally turned up to maximum do the side effects begin to become noticeable.

When used on a vocal track, the Compellor produces exactly the right degree of levelling with far fewer side effects than you could hope for using a standard dual compressor and it is, to my knowledge at least the only device that can process a complete mix, sometimes quite heavily without the discerning listener realising it.


Apart from its obvious studio uses as an ultra-sophisticated compressor, it is also a very valuable tool when used at the disk cutting stage. For broadcasting applications too, the process makes the best possible use of signal headroom, which is important when preparing commercials because anything that will push the sound 'up front' of the competition is an advantage, and in this respect, the Compellor works very well when used in conjunction with the Aphex Aural Exciter. Using this combination can produce a sound so 'up front' that it's virtually behind you.


When the Aphex Aural Exciter first appeared on the market, it seemed that Aphex were trying to sell controlled distortion in an attempt to trick the ear into perceiving a clearer sound by relying on the mysterious power of psychoacoustics. However, the Compellor follows a quite different path and represents the culmination of what must have been a lot of extensive research, whose object seems to have been to produce the most unobtrusive form of gain control possible.

All in all, the Compellor is a real masterpiece of electronic design and it seems remarkable that it can level, compress and limit, all at the same time without being noticeable. As a method of producing maximum subjective loudness, the Compellor succeeds admirably and does not cause listening fatigue. I'm certain that we'll be seeing a lot of them cropping up in studios and radio stations throughout the world.

At a retail price of £1646.59 + VAT, the Compellor isn't going to appeal to many cassette multitrack users, but for owners of a paying studio, even if only a moderately successful 8 track set-up, you could find the Compellor helps to put you into a different league.

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Publisher: Home & Studio Recording - Music Maker Publications (UK), Future Publishing.

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Home & Studio Recording - May 1985

Donated & scanned by: Mike Gorman

Gear in this article:

Studio/Rack FX > Aphex > Compellor

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Review by Paul White

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